Is it useful to paint the seams with smear tool?
It depends on how detailed your texture is. The "Smear" tool is a possibility, if you have a low-detail texture. On higher detailed texture maps it would blur out the details which in your case would be for example the hard lines between the red and the brown area on the barrel.

I prefer the Clone Brush. Use Ctrl+Leftclick to select the are to be copied from, then simply draw with Leftclick over the seam.